Most fuel ethanol is currently produced from hexose sugars in corn starch or cane syrup utilizing S. cerevisiae or Z. mobilis. However, such sugars are a relatively expensive source of biomass sugars and have competing value as foods. Alternatively, a major and cheap, renewable source of biomass is present in waste paper and yard trash from landfills, in the form of lignocellulose. Lignocellulose is primarily a mixture of cellulose, hemicellulose, and lignin. Cellulose is a homopolymer of glucose, while hemicellulose is a more complex heteropolymer comprised not only of xylose, which is its primary constituent, but also of significant amounts of arabinose, mannose, glucose and galactose. It has been estimated that microbial conversion of the sugar residues present in this abundant source of biomass (waste paper and yard trash) could provide over ten billion gallons of ethanol.
Recombinant microorganisms are known which can effectively ferment the mixture of sugars, formed by the hydrolysis of hemicellulose, into ethanol. See, for example, U.S. Pat. No. 5,028,539 to Ingram et al., U.S. Pat. No. 5,000,000 to Ingram et al., U.S. Pat. No. 5,424,202 to Ingram et al., U.S. Pat. No. 5,487,989 to Fowler et al., U.S. Pat. No. 5,482,846 to Ingram et al., U.S. Pat. No. 5,554,520 to Fowler et al., U.S. Pat. No. 5,514,583 to Picataggio, et al., and copending applications having U.S.S.N 08/363,868 filed on Dec. 27, 1994, U.S.S.N. 08/475,925 filed on Jun. 7, 1995, U.S.S.N. 08/218,914 filed on Mar. 28, 1994the teachings of all of which are hereby incorporated by reference, in their entirety. Likewise, these patents and applications describe recombinant microorganisms that can ferment the product of both the complete and partial hydrolysis of cellulose, namely glucose and the disaccharide, cellobiose into ethanol.
However, it would be highly advantageous to develop a single organism which could utilize both hemicellulose hydrolysates and cellulose hydrolysates, particularly the disaccharide, cellobiose, in the process of producing ethanol through fermentation in high yields.